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The United States has blocked Iran's pick for envoy to the United Nations, a rare diplomatic rebuke that could stir fresh animosity at a time when Washington and Tehran have been seeking a thaw in relations.

The Obama administration said Friday that the U.S. had informed Iran it would not grant a visa to Hamid Aboutalebi, a member of the group responsible for the 1979 takeover of the U.S. Embassy in Tehran. While U.S. officials had been trying to persuade Iran to simply withdraw Aboutalebi's name, the announcement amounted to an acknowledgement that those efforts had not been successful.

'Our position is that we will not be issuing him a visa.'- White House spokesman Jay Carney

"We've communicated with the Iranians at a number of levels and made clear our position on this — and that includes our position that the selection was not viable," White House spokesman Jay Carney said. "Our position is that we will not be issuing him a visa."

Aboutalebi is alleged to have participated in a Muslim student group that held 52 Americans hostage for 444 days during the embassy takeover. He has insisted his involvement in the group Muslim Students Following the Imam's Line was limited to translation and negotiation.

Hamid Babaei, a spokesman for the Iranian UN Mission, said the decision was not only regrettable but "in contravention of international law, the obligation of the host country and the inherent right of sovereign member-states to designate their representatives to the United Nations."

Exceptions for those who pose threat to U.S. security

As host country for the United Nations, the U.S. must provide rights to persons invited to the New York headquarters. However, exceptions can be made when a visa applicant is found to have engaged in spying against the U.S. or poses a threat to American national security.

Denying visas to UN ambassadorial nominees or to foreign heads of state who want to attend United Nations events in the U.S. is extremely rare, though there appears to be precedent. According to a paper published by the Yale Law School, the United States rejected several Iranians appointed to the UN in the 1980s who had played roles in the embassy hostage crisis or other acts against American citizens.

Iran's choice of Aboutalebi had pinned President Barack Obama between congressional pressure to deny the envoy entry into the U.S. and the White House's delicate diplomatic dealings with Tehran. After more than three decades of discord, U.S. and Iranian officials have started having occasional direct contact, including a phone call last fall between Obama and new Iranian President Hassan Rouhani.

The U.S. and its international partners also have reached an interim agreement with Iran to halt progress on Tehran's disputed nuclear program. Officials are in the midst of negotiating a long-term agreement to prevent Iran from building a nuclear weapon.

Officials said Undersecretary of State Wendy Sherman, the chief U.S. negotiator in the nuclear talks, informed Iranian officials involved in discussions in Vienna this week about the visa decision. The White House said it did not expect the negotiations, which are due to resume next month, to be affected by the matter.